Since a fluorine-containing ion-exchange membrane has a superior heat resistance and chemical resistance, it is employed in various applications such as an ion-exchange membrane for electrolysis used in producing chlorine and an alkali hydroxide by electrolyzing an alkali chloride, and a barrier membrane for electrolysis including ozone generation, a fuel cell, water electrolysis and hydrochloric acid electrolysis, and its new usage is further expanding.
Among these applications, the ion-exchange membrane method has recently been the most popular process for producing chlorine and an alkali hydroxide by electrolyzing an alkali chloride. The ion-exchange membrane used here is required not only to have a high current efficiency, low electrolysis voltage, and an adequate membrane strength to prevent damages during handling or electrolysis, but also to reduce the concentration of impurities, especially alkali chlorides contained in an alkali hydroxide to be produced. In order to satisfy such requirements, various proposals have been made. It is widely known that the mainstream today is a fluorine-containing ion-exchange membrane having a multi-layered structure which includes a layer containing a fluorine-containing resin having a carboxylic acid group with a high electric resistance but a high current efficiency, and a layer containing a fluorine-containing resin having a sulfonic acid group with a low electric resistance, because of being useful.
In addition, although various proposals have been made to lower electric resistance by increasing the water content of a membrane, lowering electric resistance by increasing the ion-exchange capacity of a layer containing a carboxylic acid group causes a problem that current efficiency is lowered, and at the same time, impurities in an alkali hydroxide increase. Lowering electric resistance by increasing the ion-exchange capacity of the layer containing a sulfonic acid group causes a problem that the impurities in an alkali hydroxide to be produced increases, and besides, that the strength of the membrane remarkably decreases.
Recently, as Patent Documents 1 and 2 disclose, a reduction in electrolysis voltage and improvement of membrane strength are attempted by increasing the number of layers in the membrane and specifying the water content of each layer. However, in that case, if the water content of the layer facing to the anode side is too high, not only the strength of the membrane decreases, but also the concentration of impurities contained in alkali hydroxide to be produced increases.
On the other hand, as Patent Document 3 discloses, for instance, a method is also widely known which improves the strength of the membrane by embedding a porous substrate formed of a woven fabric made from a fluorine-containing polymer such as polytetrafluoroethylene (PTFE) into the membrane.
Furthermore, a method as in Patent Document 4 has been disclosed, which improves the strength of the membrane by projecting the shape of the woven fabric made from PTFE or the like toward the anode side. However, the method forms a section surrounded by the projecting parts of the woven fabric pattern, which reduces an amount of an aqueous solution of an alkali chloride to be supplied into the anode surface of the membrane, though depending on the electrolysis condition or the structure of an electrolysis cell, and increases an amount of impurities in an alkali hydroxide to be produced. For this reason, the quality of the alkali hydroxide cannot be stabilized.
Several methods for improving the shape of the surface of a membrane in an anode side are disclosed so as to reduce the amount of oxygen in chlorine produced in the anode side during electrolysis. Patent Document 5 discloses a method of forming a groove by transferring the shape of the press roll having protruding parts to the membrane, and Patent Document 6 discloses a method of forming a groove by embedding a woven fabric into the surface of the membrane and peeling it off. However, the ion-exchange membranes obtained by these production methods has to make the thickness of the resin on the porous substrate substantially thinner, because the porous substrate made from PTFE or the like preliminarily embedded in the membrane is pushed up to the reverse side of the surface of the membrane having the groove formed thereon, which leads to the lowering of the strength of the membrane. An ion-exchange membrane receives stresses from all directions during electrolysis, so that the ion-exchange membranes obtained by those production methods show significantly reduced strength against the stress in the direction different from the direction of the porous substrate made from the PTFE or the like, for instance, the stress in 45-degree direction to the porous substrate, and consequently cannot provide a stable electrolysis performance over a long period of time. Furthermore, the ion-exchange membranes obtained by those methods do not sufficiently improve the capacity of supplying the aqueous solution of the alkali chloride into a space between an anode and the surface of the membrane, and accordingly cannot reduce the amount of impurities in the alkali hydroxide to be produced.    Patent Document 1: JP-A-63-113029    Patent Document 2: JP-A-63-8425    Patent Document 3: JP-A-03-217427    Patent Document 4: JP-A-04-308096    Patent Document 5: JP-A-60-39184    Patent Document 6: JP-A-06-279600